Manufacture of pulp by the alkaline process



Mrdl 9 1954 D. a. wEsTco-rr Erm. 2,671,727

NUFACTURE 0F PULP BY THE PROCESS Filed lay 5, 1948 my f ArroRNEYs Patented Mar. 9, 1954 MANUFACTURE F PULP BY THE ALKALINE PROCESS vDana B. Westcott and Harold L. Field, Mechanicville, N. Y., assignors to Welt Virginia Pulp and Paper Company, New York, N. Y., a corporation of Delaware Application my 5, 194s, serm No. 25,226

(ci. sz-s) schim. 1

Our present invention relates to the manufacture of pulp from wood and other suitable iibrous materials.

In general, it is the object of our invention to devise an improved process of cooking in which the raw cellulosic material will be subject to conditions of great uniformity whereby a `pulp of high uniform quality will be produced. It is a further object to provide a process oi cooking which will be more economical to operate.

In carrying out our improved process we preferably provide for the operation of a multiple of digesters served by a single accumulator for cooking liquor through which and through each digester in operation a continuous circulation of liquor is had. We further constantly maintain the active alkali concentration and temperature of the liquor entering the digesters at predetermined, preferably low optimum values preferably by continuously fortifying the liquor prior to its admission to the digesters both with active alkali and with heat. Furthermore by passing the liquor through the digesters in parallel suftlciently rapidly, the drop in active alkali concentration and that in heat content will be kept within narrow predetermined limits. When the cooking of a charge in a given digester is complete such digester is cut out of the system and blown.

Cooking a charge by bathing it in a iiowing stream of cooking. liquor which is continuously maintained within an optimum range oi' active alkali and of temperature has signal advantages, among which may be mentioned (l) a higher grade of pulp as indicated-by a higher viscosity number due to the absence of high` initial alkali concentrations as is the practice in the ordinary cooking operation and also tothe fact that there is less pulp which is either overcooked or undercooked; (2) greater use of the digester capacity inasmuch as the digester may be completely packed full of chips; (3) since the cooking liquor is continuously iortied it is possible to use the liquor until it has a greater solids content before sending it to the evaporators with substantial economies in evaporator operation; (4) the system results in cleaner blows; (5) complete submergence of the chips is had since the digester is iully charged with liquor; (6) there isno need to weigh or measure the amount of chips charged the disgester or to determine in advance the moisture contentv of the charge. l l While multiple operation of digesters has heretofore been proposed, such multiple operation has not been had so far as we are aware, in which the active alkaliconcentration oi the cooking liquor is continuously maintained within optimum limits. Furthermore while the so-called injection cooking, i. e., injecting concentrated alkali into the circulating liquor irom time to time has been proposed the advatnages o! quick heating of the chips by charging with already heated liquor available in multiple digester opration as well as other of the above advantages have not been realized.

Our invention will be best understood by reference to the following detailed description of an illustrative example taken with the annexed drawing, the single figure 'of which shows a hookup of several digesters and an accumulator adapted for carrying out the improved process.

In the drawing, there are shown digesters I, 2, 3, 4 and 5, each of which is in communication with an accumulator 6 by means of pipe 1 connected to the top of each digester by valved branch pipes la respectively; the return flow from the accumulator passes through pipe 8 and thence into oitakes Ia, Ib, Ic, Id, le, respectively leading to the bottom of digesters I to 5. Each of said ontakes has in its circuit a pump 9 and a heater I0. Each of said heaters is of the heat exchanger type in which the charge is indirectly heated by steam admitted thereto by connections not shown. Each digester has a strainer sa preierably located in the extreme top thereof.

Fresh white liquor for maintaining the active chemical concentration is admitted, preferably continuously, to the volume of liquor in circulation by means ot pipe II which joins pipe 8. Spent black liquor from the pulp washers (not shown) is similarly admitted through pipe IIa and serves to maintain the necessary volumetric level. 'I'he feed rates of both the white liquor and black liquor injected can, if desired, be made fully automatic. Since the active soda concentration is a direct function o! the electrical conductivity of the cooking liquor, the active soda conmotorized stirrer not shown whereby -to insure thorough mixing oi the added liquors. A differential pressure type level controller I 3 suitably connected to accumulator t by piping I3a may be used to measure the level in the accumulator and control this level between a suitable range by energizing a motorized valve 13b on the black liquor pipe Ila.

The air charged with the chips and any gases produced during the pulping reaction are carried by the circulating liquor from the dgesters I-5 to the accumulator 6. These gases are continuously relieved to the atmosphere from one point, line I4. A suitable bypass line I5 is had whereby the accumulator charge may be heated in heater I0 when rst starting up the system. In the hook-up shown each digester had a capacity of 808 cu. ft. and when completely iilled held from 6,000 to 10,000 pounds (depending upon its density) of chipped wood on an oven dried basis, such Wood consisting of coniferous or deciduous species or mixtures of the two and containing to 55% moisture.

In the example, the description of which now follows, approximately 10,000 pounds (O. D. basis) of mixed hardwood chips containing moisture were charged to the digester (I) and the cover bolted on. 6000 gallons (at 20 C.) of spent black liquor at the temperature available was charged to the accumulator 8 through pipe Ila. The composition of this liquor was as follows:

Grams per liter so Active soda as Naz() '1.7 Total soda as NazO 52.0 Organic matter 125.7 Total solids 192.8 Suldity (NazS as NazO) 3.0 35

The liquor charge in the accumulator 6 was heated to 150 C. by pumping out of the bottom of the accumulator through pipe 8 and pipe Ia, through heater l0 and through bypass pipe I5 back to the top of the accumulator. While this liquor was being heated, an amount of white liquor at 120 g./l. active NazO concentration was added to pipe 8 through pipe I I to raise the cooking liquor to 10 g./1. active NazO as measured by the conductivity meter. This required about 125 gallons of `:vhite liquor to effect this increase in concentration of active soda. When the liquor in the accumulator had reached 150 C. temperature and 10 g./l. active soda, the digester I was charged with this liquor by opening thc valves at the bottom and top of the digester and closing the valve in the bypass line I5. The pump 9 on digester I forced this liquor through the digester and back to the accumulator at a rate of approximately 500 gallons per minute. When the. digester was full of liquor and the liquor was flowing back to the accumulator, 5200 gallons of the original accumulator liquor charge was in the digester I and about 925 gallons in the accumulator 6.

Also, as soon as liquor from digester I began to return to the accumulator 6 and then started to flow back to the digester I, the active soda concentration fell off to about 4 g./l. or above due to dilution of the liquor with wood moisture and to reaction of the active chemical with the wood. The active soda concentration was then built up to its original 10 g./l. by injection of white liquor through pipe II. In this manner, the liquor entering the bottom of digester I was maintained at 10 g./l. active Nago concentration throughout the six hour cookafter which the digester was blown. Naturally the amount of white liquor added to maintain the NazO concentration of the entering liquor diminished as the cook proceeded.

In like manner, the temperature of the liquor entering the'digester I was maintained at a constant value of 150 C. From 45 to 60 minutes were required to bring the contents up to 150" C. Therefore, during at least five hours of the entire six hour cooking period the wood in the digester wassubjected to a cooking liquor maintained at substantially the same temperature and active chemical concentration.

A total of 2000 gallons of white liquor of g./1. active NazO was required for this six hour cook; gallons to raise the accumulator charge to 10 g./l. and 1875 gallons for injection during the cook to maintain the concentration at 1o g./1. During the cook 2680 gallons of black liquor were added at a uniform rate through pipe IIa. A total of 4555 gallons of hot white and black liquor were then added in order to have the accumulator at full charge by the end of the cook and ready to charge another cook in another digester of the battery. Similarly the accumulator will be charged suiilciently to illl any digester whenever it is ready to be cut into the system.

During the 6-hour cook, 180,000 gallons of liquor were circulated. Approximately .52 gallon of liquor at room temperature is required to cover a pound of hard wood chips on an O. D. basis. Hence the minimum amount of liquor necessary to start with is 5200 gallons for 10,000 pounds of hard wood chips. Dividing the total number of gallons pumped, 180,000 by the number of gallons necessary to cover the chips it will be seen that approximately 35 times the volume of liquor necessary to cover the chips is circulated during the cook.

The pulp so produced from mixed hard woods had the following properties:

Permanganate number 13.8 Viscosity1 cp 5,550 Screenings on basis of screened pulp obtained,

less than 5.4%

lTappi: A 1.25% solution of the pulp in 0.5 molar cupriethylene diamine is first formed and its viscosity then measured by the falling ball method.

A total of 0.2 pound (91 grams) of active soda. per pound of O. D. wood was applied during the cook, this soda being added as a kraft white liquor containing 116 g./l. NaOH and 38 g./l. NazS. As is customary both the sodium hydroxide and the sodium sulfide are expressed in terms of NazO and the total of the two constitute the active NazO concentration in the cooking liquor. The high viscosity of the unbleached pulp was evidence that the cellulose had undergone relatively little degradation and that the pulp would have a high strength (we have found that if the viscosity of the unbleached pulp falls below 2000 centipoises, the strength is inferior. Otherwise expressed, the fall of strength with fall of viscosity proceeds at an accelerated rate below 2000 cp.). In the present process the high viscosity obtained is attributable to the low active NazO concentration of the cooking liquor, to the comparatively low temperature of the cook and the uniformity of the cooking conditions. If desired the temperature of the cook may be raised from a minimum, say of C. to 175" as the upper limit, with a. shortening of the time of cook, but the pulp will have a decreased viscosity. Similarly the quality of the pump will undergo a change when the concentration of the active alkali is increased. Whatever change is brought about by variation of temperature and concentration of chemicals the pulp produced is of uniform quality. For best results we have found that the active alkali concentration of the cooking liquor as it enters the digester should be not .substantially less than 10 g./l. and not substantially more than 20 g./I.

It will be noted that although the quantities are here given for the sake of completeness, it is not necessary to weigh the chips added to the digester nor is it necessary to know their moisture content, since the liquor entering the digester will be maintained at the` same concentration of active chemicals.

When operating the battery-of five digesters shown, after No. l digester has been started, digester No. 2, after having been nlled with chips is started up after a suitable interval, say 81 minutes from the time that digester No. l was started by pumping thereto liquor through the pipe Ib. 'I'he black liquor injection rate must be increased enough so that the accumulator is full and ready to charge in 81 minutes, using a 6-hour length of cook. Also suflicient white liquor will be added to the accumulator through pipe li to maintain the concentration at 10 g./l. NaaO or other predetermined value. Similarly, each of the other digesters 3, 4 and 5 will be filled with chips and cut into the system each one following the other after the same time interval.

When a newly filled digester is cut into the system the liquor which is first p assed therethrough is naturally cooled considerably and diluted by the moisture in the chips. However, this dilution and loss of heat will be easily made up by reason of the fact that the diluted liquor is mingled with the liquor in circulation from the other digesters which in effect function as accumulators. Likewise the steam demand of the system is made much more uniform than if a single digester were being employed.

If desired the process may be carried out with the combination of an accumulator and a single digester with, however, corresponding loss of operating emciency.

We claim:

1. The method of cooking wood to obtain pulp therefrom, which comprises the steps of filling a plurality of digesters with wood chips, maintaining a continuous flowing stream of alkali cooking liquor through said digesters in parallel and through an accumulator, the wood in each digester being maintained submerged in a flowing body of liquor, the liquor having a temperature in the range of about 145 C. to about 175 C. and having an effective Naz'O content in the range of from about ten grams per liter to about twenty grams per liter, substantially continuously assay'm g the stream of liquor leaving the accumula `r on its way to said plurality of digesters for effective alkali content and temperature, restoring depleted NazO and heat values to said liquor in response to said assay to maintain the same within the said ranges, blowing each of said digesters sequentially after completion of the cook therein while maintaining the stream through the remaining digesters and accumulator, filling each blown digester with Wood chips and connecting the same to the stream, subjecting the wood in the last named digester to the action of the stream at the said cooking temperature and alkali content thereby quickly bringing the same to cooking temperature, and accomplishing the cooking thereof as above set forth.

2. The method of cooking wood to obtain pulp therefrom, which comprises the steps of nlling a plurality of digesters with wood chips, maintaining a continuous ilowing stream of alkali cooking liquor through said digesters in parallel and through an accumulator, the wood in each digester being maintained submerged in a flowing body of liquor, the liquor having a temperature in the range of about C. to about 175 C., the said stream having an effective NazO content in the range of from about ten grams per liter to about twenty grams per liter when entering the digester, the rate of flow of said stream being such that said NazO content is maintained at not less than four grams per liter at the outlet end of the digesters, substantially continuously assaying the stream of liquor leaving the accumulator on its way to said plurality of digesters for effective alkali content and temperature, restoring depleted NazO and heat values to said liquor in response to said assay, blowing each of said digesters sequentially after completion of the cook therein while maintaining the stream through the remaining digesters and accumulator, filling each blown digester with wood chips and connecting the same to the stream, subjecting the wood in the last named digester to the action of said stream atthe said cooking temperature thereby quickly bringing the same to cooking temperature, and accomplishing the cooking thereof as above set forth.

3. The method of cooking wood to obtain pulp therefrom, which comprises the steps of illling a plurality of digesters with wood chips, maintaining a continuous flowing stream of alkali cooking liquor through said digesters in parallel and through an accumulator, the wood in each digester being maintained submerged in a owing body of liquor, the liquor having a temperature in the range of about 145 C. to about 175 C. and having an effective Nago content in the range of from about ten grams per liter to about twenty grams per liter, restoring depleted alkali and heat to said stream of liquor leaving the accumulator on its way to said plurality of digesters, blowing each of said digesters sequentially after completion of the cook therein while maintaining the stream through the remaining digesters and accumulator, filling each blown digester with wood chips` and connecting the same to the stream, subjecting the wood in the last named digester to the action of the stream at the said cooking temperature and alkali content thereby quickly bringing the same to cooking temperature, and accomplishing the cooking thereof as above set forth.

4. The method of cooking wood to obtain pulp therefrom, which comprises the steps of filling a plurality of digesters with wood chips, maintaining a continuous flowing stream of alkali cooking liquor through said digesters in parallel and through an accumulator, the wood in each digester being maintained submerged in a owing body of liquor, the liquor having a temperature in the range of about 145 C. to about 175 C., the said stream having an effective NaaO content in the range of from about ten grams per liter to about twenty grams per liter when entering the digester, the rate of ow of said stream being such that said Nano content is maintained at not less than four grams per liter at the outlet end of the digesters, restoring depleted heat values and alkali content to the stream of liquor leaving the accumulator on its way to said plurality of digesters, blowing each of said digesters sequentially after completion of the cook therein while maintaining the stream through the re- 7 maining digesters and accumulator, lling each blown digester with wood chips and connecting the same to the stream, subjecting the wood in the last named digester to the action of said stream at the said cooking temperature thereby quickly bringing the same-to cooking temperature, and accomplishing the cooking thereof as above set forth.

5. The method oi claim 3 in which the cooking liquor is a kraft liquor.

6. The method of claim 3 in which the ow o! the liquor is upwardly through said digesters.

7. The method of claim 4 in which the cooking liquor is a kraft liquor.

8. The method of claim 4 in which the iiow of the liquor is upwardly through the digesters.

DANA B. WESTOO'TT. HAROLD L. FIELD.

8 References Cited in the nle of this patent UNITED STATES PATENTS Number Name Date 882,790 Muntzlng Mar. 24, 1908 1,690,954 Spencer Nov. 6, 1928 1,784,849 Richter Dec. 16, 1930 1,887,899 Bradley et al Nov. 15, 1932 1,918,459 Dunbar et al July 18, 1933 2,190,193 Richter Feb. 13, 1940 FOREIGN PATENTS Number Country l Date 441,890 Canada June 3, 1,947

OTHER' REFERENCES Wells et al. article in Paper Trade Journal, pp. 30-36, November 9, 1944.

' Wells article in Paper Trade Journal, pp. 42-46, October 30, 1947. 

1. THE METHOD OF COOKING WOOD TO OBTAIN PULP THEREFROM, WHICH COMPRISES THE STEPS OF FILLING A PLURALITY OF DIGESTERS WITH WOOD CHIPS, MAINTAINING A CONTINUOUS FLOWING STREAM OF ALKALI COOKING LIQUOR THROUGH SAID DIGESTERS IN PARALLEL AND THROUGH AN ACCUMULATOR, THE WOOD IN EACH DIGESTER BEING MAINTAINED SUBMERGED IN A FLOWING BODY OF LIQUOR, THE LIQUOR HAVING A TEMPERATURE IN THE RANGE OF ABOUT 145* C. TO ABOUT 175* C. AND HAVING AN EFFECTIVE NA20 CONTENT IN THE RANGE OF FROM ABOUT TEN GRAMS PER LITER TO ABOUT TWENTY GRAMS PER LITER, SUBSTANTIALLY CONTINUOUSLY ASSAYING THE STREAM OF LIQUOR LEAVING THE ACCUMULATOR ON ITS WAY TO SAID PLURALITY OF DIGESTERS FOR EFFECTIVE ALKALI CONTENT AND TEMPERATURE, RESTOR- 